The proposed experiments will investigate the role of statistical learning in the acquisition of syntactic categories such as "noun" and "verb". The role of syntactic category knowledge in language acquisition is pervasive;for example, category knowledge both facilitates learning grammatical patterns and facilitates early word learning. While many aspects of language acquisition rely on syntactic category knowledge, the process by which these categories are learned is unclear. The goal of the proposed research is to test the hypothesis that statistical learning plays a role in acquiring syntactic categories. Words from different syntactic categories are distinguishable by statistical cues, such as their distributional properties (i.e., cooccurrence with other words), and their phonological properties (i.e., their sound structure), the first specific aim is to assess infants'sensitivity to these cues in the acquisition of noun and verb categories. If words'statistical properties are used in category learning, infants should be able to use these properties to identify novel category members in the early stages of category acquisition. Moreover, infants'ability to determine the category of novel words should reflect the degree to which they conform to the predominant statistical characteristics of the category, and training on these statistical properties should facilitate their use at younger ages. Infants'ability to categorize novel words will be assessed by testing whether they are mapped to object or action referents. The second specific aim is to test infants'ability to integrate statistical and semantic information in category learning. Acquisition of syntactic categories encompasses learning about both statistical properties and semantic properties, and these cues are correlated in natural language. If infants can integrate these cues, correlations between them should facilitate learning. Infants will be familiarized with an artificial language containing novel categories, and the correlations between statistical and semantic cues will be manipulated. If infants can integrate these cues, they should be better able to detect commonalities in the meanings of words within a category, and to learn the grammatical structures in which these words can occur, when cues are correlated than when they are not. Future studies will test children with Specific Language Impairment and other developmental language disorders (e.g., late talkers), who exhibit impaired word learning abilities and reduced sensitivity to distributional cues identifying syntactic categories. Testing children on these tasks will address the extent to which impaired statistical learning is a contributing factor in these disorders.